It's tough. Logbook?
Keep track of the type, duration, and intensity of exercise
in a log book.
Use this to track your progress/improvement. Depending on your type of exercise, make one or more periodic adjustments weekly: may increase speed, increase duration, increase incline, amount of weight, number of weight sets, swim laps, etc.
A log book is more helpful than it sounds.
Also, use a buddy- motivates both.